1. Field of the Invention
The present invention relates to a machine for grinding cylindrical bearing surfaces on parts, in particular journals and crank pins on crankshafts, using an abrasive belt, the machine including a support on which three abrasive belt clamping members are mounted in a triangle so that they can move in directions that intersect on the axis of the bearing surface to be ground.
2. Description of the Prior Art
Superfinishing machines of the above type are known in the art, for example from the documents FR-A-2 634 877, U.S. Pat. No. 5,522,762 and U.S. Pat. No. 5,651,719. As described in the above documents, the abrasive belt clamping members carry abrasive belt applicator shoes in the form of jaws which can be very hard and subtend a larger or smaller circumferential angle, which in particular makes it possible to correct any shape defects caused by preceding grinding operations. On the other hand, the clamping members with their dedicated applicator shoes are designed for a very specific diameter, which means that a machine equipped with these clamping members can be used only to grind specific parts, for example crankshafts of one design of engine. However, for reasons of flexibility, it would be desirable to be able to use the same machine to grind crankshafts for several designs of the same type of engine, for example, or even crankshafts for different types of engine, in particular engines in which the journals and/or crank pins may have different diameters.
The problem stated above in connection with the diameter of the bearing surfaces to be ground can also arise in connection with the width of the bearing surfaces.
What is more, the prior art machines are not entirely satisfactory for grinding bearing surfaces of cast iron crankshafts because the applicator shoes used in these machines are not able to detach graphite nodules from the bearing surfaces because of their hardness and their circumferential angles of contact with the bearing surfaces.
The present invention relates to an abrasive belt grinding machine that is distinguished by a very high degree of flexibility in relation to the diameter of the bearing surfaces that can be ground using the same abrasive belt clamping members. The invention also relates to an abrasive belt grinding machine that is distinguished by a very high degree of flexibility in relation to the width of the bearing surfaces that can be ground using the same abrasive belt clamping members and the same abrasive belts. The invention also relates to an abrasive belt grinding machine which is distinguished by improved efficiency in grinding cast iron crankshaft bearing surfaces.
The invention provides a machine for grinding cylindrical bearing surfaces on parts, in particular journals and/or crank pins on crankshafts, using an abrasive belt, including three abrasive belt clamping members mounted in a triangle and mobile in directions that converge toward the axis of the bearing surface to be ground, wherein each clamping member carries at least one applicator shoe conformed to press the abrasive belt against the bearing surface along a surface essentially limited to a generatrix parallel to the axis of the bearing surface.
The shoe can advantageously have a cylindrically curved applicator surface, preferably with a radius of curvature less than the radius of curvature of the bearing surface and more particularly less than half the radius of curvature of the bearing surface.
Each abrasive belt clamping member can advantageously carry two applicator shoes spaced in the circumferential direction, preferably by a distance that substantially corresponds to the distance between the successive shoes of two consecutive clamping members, for the average of the extreme diameters of bearing surfaces that can be ground on the same machine.
Each shoe is preferably made of a material such as an elastomer having a Shore hardness less than or equal to 100.
If the bearing surface has been rough-ground with a curvature that must be conserved on superfinishing, it is possible for each abrasive belt applicator shoe to have, along the axis of the bearing surface to be ground, two end parts of higher hardness on respective opposite sides of an intermediate part of lower hardness. For example, the end parts can have a Shore hardness of the order of 95 and the intermediary part a Shore hardness of the order of 65.
To be able to machine bearing surfaces with different widths on the same machine without having to change the abrasive belt, the clamping members and the applicator shoes mounted on those members, it is advantageous to make the length of the shoes and the width of the abrasive belt less than the width of the narrowest bearing surface and to cause the support on which the abrasive belt clamping members are mounted to oscillate at a low speed during the grinding of the bearing surface and over a greater or lesser stroke, which is manifested in the form of a tacking movement of the abrasive belt relative to the bearing surface of the part which is driven in rotation in the usual way. This tacking movement imparted to the abrasive belt is independent of the oscillatory movement at high speed and over a relatively small stroke usually imparted to the part during superfinishing.